A filter is a main component in an electronic device, which can filter various signals, so as to ensure operation stability and reliability of a circuit. In a filter circuit of a direct current power supply, a combinatorial circuit of a discrete X capacitor and a discrete Y capacitor is generally used as a filter to eliminate noise in a power supply circuit, such as clutter.
In the prior art, when filtering processing is performed for a direct current power supply, a discrete capacitor component is generally adopted. Specifically, an X capacitor is connected between a power cable and an earth return or between a power cable connected to a positive electrode and a power cable connected to a negative electrode, and is used to eliminate differential mode noise in a circuit. A microfarad-level thin film capacitor or a multilayer ceramic capacitor is generally used as the X capacitor. Meanwhile, a Y capacitor is connected between a power cable and a ground cable. Two capacitors are connected in series to serve as a Y capacitor, which then bridges the power cable and ground, and is used to eliminate common mode noise in a circuit. Because a phase of transmitted common mode noise is the same, connection symmetry of a Y type circuit is quite high, an earth leakage current is small, and a withstand voltage is relatively high. A Y capacitor generally uses a nanofarad-level ceramic capacitor. According to a function of a capacitor, the X capacitor which can eliminate differential mode noise may also be called a differential mode capacitor, and the Y capacitor which can eliminate common mode noise may be called a common mode capacitor. When a discrete capacitor component is used to connect a circuit, a large number of capacitor components are used and occupy large space. In addition, a high requirement is imposed on a location and accuracy of component disposition, and a filtering effect is relatively poor.
Therefore, in the prior art, a filter with a multi-layer structure is also put forward, which integrates a differential mode capacitor and a common mode capacitor, and includes two external common electrode layers and at least two inner electrode layers disposed between the two external common electrode layers. In addition, a common electrode layer is disposed between each two inner electrode layers, each electrode layer is connected to an external electrode disposed at an end of the filter, and an external circuit may connect to an inner electrode layer by using the external electrode. In this filter structure, a capacity of the formed differential mode capacitor is relatively small, and a differential mode filtering capability is relatively poor. Therefore, the filter structure can be used only in a high-frequency filter circuit, and cannot be applied to filtering of relatively low-frequency noise such as that of a direct current power supply.
To sum up, because a capacity of a differential mode capacitor is relatively small, an existing filter with a multi-layer structure cannot be applied to filtering processing of a direct current power supply such as that of low-frequency noise.